The next time you feel angry at a friend who has let you down, or grateful toward one whose generosity has surprised you, consider this: you may really be bargaining for better treatment from that person in the future. According to a controversial new theory, our emotions have evolved as tools to manipulate others into cooperating with us.

Until now, most psychologists have viewed anger as a way to signal your displeasure when another person does you harm. Similarly, gratitude has been seen as a signal of pleasure when someone does you a favour. In both cases, emotions are seen as short-term reactions to an immediate benefit or cost.

But it’s more cunning than that, says John Tooby, an evolutionary psychologist at the University of California, Santa Barbara. Anger, he says, has as much to do with cooperation as with conflict, and emotions are used to coerce others into cooperating in the long term. (…)

All this suggests that anger and gratitude – and perhaps other emotions, too – may be tools for turning up a partner’s mental cooperation control dial.